A critical dimension (CD) of a semiconductor device refers to a feature that has a direct impact on the device's performance or its manufacturing yield. Therefore, CD's must be manufactured or controlled to tight specifications. Examples of CD's include gate length, gate width, interconnect line width, line spacing, and line width roughness (LWR), to list a few examples. Semiconductor devices are very sensitive to these dimensions and even minute variations can lead to large degradations in their performance, device failure, or manufacturing yield.
As integrated circuit (IC) feature sizes of semiconductor devices continue to shrink, manufacturers face ever decreasing process windows and tighter tolerances. This has dramatically raised the accuracy and sensitivity requirements for CD metrology tools as well as the need for non-destructive measurement sampling early in the manufacturing cycle with minimal impact to productivity of the semiconductor device manufacturing plant or fab. Furthermore, because of the high-aspect ratio features used in IC's today, the need for three dimensional (3D) profile information on device structures, including sidewall angle, and top and bottom dimensions, has become critical. An exemplary driver of this need is the 3d gate structures used in new transistor designs. Consequently, the ability to measure the 3D profile provides far more valuable information than the conventional two dimensional line width and spacing CD information.
Historically, CD measurements were made with optical microscopes, but as dimensions shrank to sub-micron scales, electron microscopes (CD-SEM) became an alternative solution in recent years. Another solution is an optical technique developed by International Business Machines Corporation, Inc. (IBM) and later commercialized by Nanometrics Incorporated using scatterometry to make CD measurements with visible light. Taking advantage of low-cost computing power, this method measures the diffraction patterns from a sample illuminated with visible light and determines the CD parameter using a combination of look-up data library and an inverse computing model based on the rigorous coupled wave analysis (RCWA). Often referred to as optical critical dimension (OCD) metrology, it is a non-destructive and high-throughput technique that has been widely used for process control. Other techniques such as atomic force microscopy (AFM) have also been applied to CD measurement, but wide use of AFM, for example, has been limited by its measurement speed and probe size.
As IC feature dimensions approach 32 nanometers (nm), the metrology requirements push these technologies to their fundamental limits. The disadvantages of CD-SEM have become more critical: 1) the well known charging problem limits the achievable resolution for IC metrology applications; 2) radiation damage induced dimensional shrinking of resists (note: this is particularly problematic with some 193 nm photoresists, for example, that shrink when exposed to electron beams, leading to “CD slimming”); 3) incompatibility with some low-k dielectrics; and 4) CD-SEM is essentially a surface technique making it difficult to measure 3D profiles. Similarly, OCD faces a number of fundamental difficulties: (1) its long wavelength is significantly larger than the device feature size and therefore does not provide a simple and direct measurement; 2) it requires extensive modeling and interpolation, thus compromising the measurement sensitivity. Moreover, over the last decades, use of shorter and shorter wavelengths has been necessitated by the reduction of circuit feature size. Currently the most advanced OCD system uses the deep ultraviolet (DUV) and further incremental reduction in wavelength is not practical because of the extremely low transmission of shorter wavelength radiation in solids or even in low vacuum. This results in numerous problems, including low probing depth, lack of suitable optics, and stringent vacuum requirements. These fundamental limitations have made it practically impossible to extend these existing technologies to meet the critical dimensional control requirements of next generation IC fabrication.
Recognizing the limitations of the existing CD metrology tools, CD metrology and 3D structure characterization have been identified as a Grand Challenge in the International Roadmap for Semiconductors (ITRS) published by SEMATECH. Nearly all existing CD metrology technologies show significant difficulties for node sizes approaching 45 nm and significant research effort is expected to determine a viable solution for future generation ICs.
Dr. Wenli Wu, et. al. at the National Institute of Standards and Technology (NIST) has recently pioneered a new solution for future CD metrology based on a small angle x-ray scattering technique. This technique can be considered as an extension of the current OCD metrology technique. This Critical Dimension Small Angle X-Ray Scattering Technique (CD-SAXS) uses x-rays with wavelengths much smaller than circuit feature dimensions expected in many future IC generations. By illuminating a sample with a monochromatic x-ray beam and measuring its diffraction pattern(s) with a spatially resolved detector, the structure of the sample with its critical dimensions can be analyzed using well established techniques. This technique is also well suited to determine 3D structures consisting of the same materials as well as complex material structures such as the diffusion barrier coated on dielectrics used in the copper damascene process, a capability analogous to the x-ray crystallography technique that has achieved great success in understanding complex protein structures by examining their diffraction patterns.
Using the synchrotron radiation source at Argonne National Laboratory, Wu, et. al. have provided a first demonstration of the effectiveness of the CD-SAXS system to measure critical dimensions such as pitch, width, and height. They compared CD-SAXS measurement with CD-SEM using programmed LWR structures having trapezoidal profiles.
In summary, the pioneering works by Dr. Wenli Wu, et. al. shows that the CD-SXAS offers the following advantages. First, the x-ray wavelength is significantly shorter than the circuit feature size making the technique suitable for many future IC manufacturing generations providing good diffraction patterns and sensitivity. Second, simple data analysis is adequate to reconstruct the device structure (2D and 3D) because of the lack of multiple scattering. Third, the mass is measured instead of just geometry allowing a determination of line height roughness. Until now, the main drawback noted by Wu, et. al. is that the measurements using a laboratory x-ray source requires several hours per site. While that those time frames make the technique useful as a research and development tool, significant throughput improvement is required to make the CD-SAXS practical to meet the grand challenges for in-line IC metrology applications.